(1) Field of the Invention:
This invention relates to a discharge lamp suitable for use as a light source, for example, in projectors.
(2) Description of the Related Art:
In discharge lamps of a large current capacity, the rod-sealing system, in which an electrode rod is directly sealed in an envelope without using a metal foil, is often used. In a rod-sealing type discharge lamp, its light-emitting tubing is usually equipped with an envelope having a bulged portion defining a discharge space and side tube portions extending from both sides of the bulged portion. Electrode rods are provided in such a manner that they hermetically extend through the respective side tube portions into the bulged portion. The side tube portions of the envelope have a graded seal structure constructed by joining a plurality of glass members, which are different in thermal expansion coefficient, in order of their thermal expansion coefficients.
However, since the discharge lamp equipped with the side tube portions of such graded seal structure tends to crack at the side tube portions due to overheating of sealed portions, it is necessary to reduce the temperature of the sealed portions.
A conventional discharge lamp is illustrated in FIG. 2. In this figure, numerals 1 and 2 indicate an envelope and a bulged portion defining a discharge space 3, respectively, while numerals 4, 5, 6, 10, 11, 20 and 30 designate a cathode, an anode, a stem, a light-emitting tubing, a side tube portion, an electrode rod and a base, respectively. In the conventional discharge lamp, a radiating space 13 is defined by the base 30 fitted and fixed on the side tube portion 11 on the outside of the end of the side tube portion 11. Radiating apertures 61 for communicating with the radiating space 13 are defined through a peripheral wall of the base 30. Namely, the conventional discharge lamp has been designed to cool its sealed portions by air flowing through the radiating apertures 61.
The above base 30 is fixed on the side tube portion 11 with a cement B filled in a gap between the base 30 and the outer peripheral surface of the side tube portion 11. Specifically, this fixing process has been conducted in the following manner. Namely, a heat-resistant liquid cement is applied on the inner peripheral surface of the opening portion of the base 30 having the radiating apertures 61 in advance. The thus-coated base 30 is fitted on the side tube portion 11 in such a manner that the radiating space 13 is defined on the outside of the end of the side tube portion 11. The cement is then dried to fix the base 30 on the side tube portion 11.
However, a process for fitting the base 30 on the side tube portion has been found to involve a problem that the liquid cement is pressed by the outer peripheral surface of the side tube portion 11 and the like, so that the cement B flows out into the radiating space 13 and the amount of the cement present between the inner peripheral surface of the base 30 and the outer peripheral surface of the side tube portion 11 is hence too reduced to obtain insufficient bonding strength.